1. Field of the Invention
This invention relates to a dispenser cathode suitable for use in a cathode ray tube and a method of making the dispenser cathode.
2. Description of the Prior Art
As televisions become larger in size and capable of high resolution images, a dispenser cathode in operation with the cathode ray tube of such televisions demands a high performance with continuous electron emission at high current densities without compromising the effective life span of the cathode.
A conventional dispenser cathode as shown in FIG. 1 typically comprises porous tungsten matrix 1 impregnated with barium calcium aluminate (BaO--CaO--Al.sub.2 O.sub.3). Matrix 1 is contained in reservoir 2 made of refractory metals such as molybdenum and tantalum. The dispenser cathode also includes cylindrical sleeve 3 in which reservoir 2 is fitted at the upper part thereof and heater 4 at the lower part of sleeve 3. The tungsten matrix is formed by compacting a quantity of tungsten powders and sintering the mass at a temperature of 1900 .degree.-2300.degree. C. Reservoir 2 having matrix 1 held therein is inserted into sleeve 3 so that it forms flush with its uppermost edges and welded thereto by a conventional laser welding technique. A layer of a platinum-group metal, e.g., Ir, Os, Ru and Re, might be coated on the emissive surface of matrix 1 to improve its electron emissivity.
Although this dispenser cathode is capable of emitting high currency density, it operates at high temperature with low heat transfer efficiency. This is because heat from heater 4 is not effectively transferred to the electron emissive surface of matrix 1 due to reservoir 2 of the dispenser cathode structure which forms a gap between matrix 1 and reservoir 2. Furthermore, the gap may grow in proportion to the difference in the rates of heat expansion associated with the reservoir and matrix.